Photovoltaic cells capable of converting sunlight directly to electrical energy have been commonly used for decades. Most present-day photovoltaic devices employ single cells, which are satisfactory for low energy consumption devices, but inherently have a high cost/output watt ratio. Polycrystalline photovoltaic cells can be manufactured at a much lower cost than single crystal cells, but have generally reduced efficiencies. Nevertheless, a great deal of research has been and is continuing with respect to the development of higher efficiency polycrystalline photovoltaic products which utilize thin layers of particular chemical compositives to form the n-type material and the p-type material of the photovoltaic junction. Much of this technology has detracted from the advantages of low cost in favor of increased efficiency. While high efficiency is obviously desired, low material and manufacturing costs are critical if photovoltaic technology is to have increased practical significance. Low cost polycrystalline cells can currently provide relatively low cost electrical power at remote locations, such as telecommunication stations, agricultural water pumping sites, remote villages and portable housing facilities. Improvements in such technology may well result in future photovoltaic power plants which compete with conventional hydrocarbon consuming plants. The present invention is directed to such polycrystalline photovoltaic technology and, most importantly, is directed to improvements in photovoltaic technology which will result in increased efficiency but do not significantly increase the material or manufacturing costs of the photovoltaic cells.
A low cost photovoltaic cell with a p-type Cu.sub.x S layer and an n-type CdS layer disclosed in U.S. Pat. Nos. 3,902,920 and 4,086,101. Such a cell may be formed on a glass substrate according to the techniques disclosed in U.S. Pat. No. 3,959,565. Improved CdS film for such a cell is the subject of U.S. Pat. No. 4,095,006, and residual chlorides in a CdS layer are disclosed in U.S. Pat. No. 4,178,395. The n-type and/or the p-type polycrystalline layers of the photovoltaic cell may be regrown as disclosed in U.S. Pat. No. 4,362,896. Improved equipment for forming such a low cost cell is disclosed in U.S. Pat. Nos. 4,224,355, 4,239,809, 4,338,078 and 4,414,252. Instead of utilizing a spray pyrolysis technique, the n-type polycrystalline layer or the p-type polycrystalline layer may be formed according to a technique which utilizes compression preceding regrowth, as disclosed in U.S. Pat. No. 4,735,909. After the various polycrystalline layers have been formed on a substrate, individual cells may be formed and interconnected in a series arrangement according to U.S. Pat. Nos. 4,243,432 and 4,313,022. Various cell or panel encapsulation techniques have been devised, and a low cost yet reliable panel encapsulation technique is disclosed in U.S. Pat. No. 5,022,930. The series connected cells on a glass substrate form a photovoltaic panel, and various photovoltaic panels may be assembled in a module according to the techniques of U.S. Pat. No. 4,233,085.
Various materials have been suggested for forming the junction layers of the polycrystalline photovoltaic cell. A great deal of research has been expended with respect to polycrystalline silicon photovoltaic cells, in part because early tests resulted in reasonably high efficiency for these cells. Copper indium diselenide polycrystalline cells have been developed, and show some promise of improved efficiencies. Cadmium telluride cells, and particularly such cells wherein the cadmium telluride is the p-type material, nevertheless are generally considered to offer the lowest cost production potential. It must be recognized, of course, that the commercial cost of a photovoltaic cell does not only consist of the material and manufacturing cost of active junction layers which convert sunlight into electrical energy, since the material and manufacturing costs of the substrate, the appropriate electrode configuration of the cells and for interconnecting the cells, and the encapsulation mechanism must all be considered in analyzing the overall cost of the photovoltaic product. The ideal solution to one problem, i.e., high efficiency and low cost junction formation, must also be compatible with the technology used to achieve a desired overall photovoltaic product at the desired cost/output ratio.
Cadmium telluride photovoltaic cells offer an advantage of relatively low costs. Moreover, cadmium telluride cells may be manufactured with low-cost deposition equipment for applying the CdTe film layer, as described in the previously-referenced patents, and this cell does not require extremely close quality control to obtain reasonable efficiency. Various materials have been proposed for the n-type layer for forming the photovoltaic junction with the cadmium telluride layer. Cadmium sulfide has been considered a suitable n-type material for such a cell, and particularly for a low cost cadmium telluride cell, since it also has a relatively low manufacturing cost and may be deposited at atmospheric pressure utilizing low-cost deposition equipment. U.S. Pat. No. 4,568,792 discloses various types of cadmium telluride cells, and notes that CdS is an advantageous n-type material because of its wide band gap. Various materials have been suggested for "doping" the p-type cadmium telluride layer, while the n-type layer may be oppositely doped. U.S. Pat. No. 4,705,911 discloses a CdS/CdTe solar cell, wherein an oxygen-releasing agent is provided to minimize reduction of the p-type characteristics of the cadmium telluride.
In spite of its low-cost per watt of useful power output, CdS/CdTe cells have not been widely accepted because of continuing relatively poor efficiency. One long-recognized reason for such poor efficiency relates to the construction of the cells and the optical band gap of the CdS layer. To produce electrical energy, light must reach the junction of the cell, i.e., the CdS/CdTe interface. The CdTe layer generally serves as the light absorber, and in a typical structure the CdS serves as a heterojunction partner and an optically transmissive layer. This design may result in a "backwall" cell, wherein light passes through a CdS layer which is deposited on the CdTe layer, which was previously deposited on an opaque substrate. Alternatively, an "inverted backwall" design may be used, wherein light first passes through a highly-transmissive substrate (glass) then through the CdS layer which was deposited over the substrate, to reach the junction formed when the CdTe layer is deposited on the CdS layer. A "front wall" cell may be formed utilizing a CdS/CdTe design, wherein the CdTe layer is deposited over a CdS layer, which was previously deposited on an opaque substrate, or an inverted "front wall" cell formed by depositing the CdTe layer on a glass substrate, with the CdS layer then applied on the CdTe layer. While the backwall or inverted backwall design of a CdS/CdTe cell is preferred, a significant quantity of energy is lost in the CdS layer, since cadmium sulfide does not pass optical energy with a wavelength shorter than approximately 520 nm unless the film is very thin. The CdS layer must be continuous to provide the desired junction and, most importantly, to prevent shorts to the electrode layer. To utilize the desired low-cost deposition equipment, the CdS layer has necessarily been of a thickness such that very little of the optical energy with wavelengths less than 520 nm reached the junction and produced useful energy. U.S. Pat. No. 4,251,286 discloses utilizing a zinc sulfide blocking layer to prevent electrical shorts, but this technique is expensive and introduces additional complexities which are believed to have a significant adverse affect on the life of the cell. U.S. Pat. No. 4,598,306 also discloses the use of a barrier layer for preventing electrical shorts between the active photovoltaic layers and an electrode. The barrier layer operates as a series resistor to limit current flow through the otherwise short circuit current path. U.S. Pat. No. 4,544,797 discloses another technique for preventing short circuits by passivating areas of a first conductive electrical contact which are not covered by the adjoining n-type or p-type material. This passivating step may be performed by immersing the device in ammonium sulfide to convert a silver metallic layer at the location of pinholes to an n-conductive Ag.sub.2 S material. This procedure is similarly costly and again introduces additional chemicals into the cell formation process which are not desired. An article entitled "Properties of the Screen-Printed and Sintered CdTe Film Formed on a CdS Sintered Film" in Technical Digest of the International PVSEC, Vol. B-III (1987), p. 5 suggests that screen-printed CdS/CdTe cells may have improved longer wavelength sensitivity due to the formation of mixed CdS.sub.x Te.sub.1-x crystals during sintering of the CdTe.
The disadvantages of the prior art are overcome by the present invention, and an improved photovoltaic cell and method of forming a photovoltaic cell are hereinafter disclosed. A photovoltaic panel comprising a plurality of cells formed according to the techniques of the present invention has the desirable benefit of low material and manufacturing costs, yet produces a considerable increase in photovoltaic conversion efficiency compared to prior art devices which have not included this technology, thereby resulting in a photovoltaic panel having relatively low overall cost per watt of useful power output.